The invention refers to a method for fractionation of substances by crystallization in which the raw product, particularly a mixture of fatty acid esters, is mixed with a solvent and the solution obtained thereby is circulated in a circuit with crystals being formed in the solution by gradually lowering the temperature.
The method is particularly suitable for fractionation of mixtures of fatty acid esters and other slowly crystallizing mixtures of substances and/or mixtures of substances difficult to filtrate. Natural fats or oils are mixtures of mono-, di- and triglycerides of saturated and unsaturated fatty acids. For their further use the problem occurs to fractionate the natural raw product into two or more fractions having melting points differing considerably from each other; e.g. into fat and oil fractions. The part with the highest melting point is often not suitable for use in foodstuff, but is only usable for industrial purposes. For reasons of economy this fraction should be separated in highly concentrated form.
Normal palm oil has a melting point of approximately 40.degree. C. (Celsius). To obtain with a good yield an oil fraction with a melting point lower than 10.degree. C. is a problem which is not yet solved satisfactorily for bulk processing.
A similar fractionating problem is posed after glycerides have been transformed by ester interchange into methyl fatty acid esters or other esters.
In food technology, and in chemical technology fractionation of mixtures of substances by employing the different crystallization temperatures of the components is a generally employed method. This method is gentle from the point of view of operating temperatures and offers in many cases substantial advantages compared to other possible methods, e.g. distillation. For the fractionation of mixtures of chemically similar substances, crystallization is very often the preferred fractionating method for reasons of its economy. This is also true for the fractionation of mixtures of fatty acid esters. However, in this case the use of crystallization is particularly difficult, because the components capable of crystallization tend strongly to undercooling and after initiation of crystallization form crystal mixtures with low selectivity. Further, the separation of the crystals from the liquid phase with known means such as sedimentation or filtration is expensive and of low selectivity. By adding a solvent the conditions can be slightly improved, but the basic problems remain. The use of crystallization causes additional problems when the substance to be crystallized may form different crystal modifications on lowering of the temperature.
In a known method for separating of mixtures of glycerides of fatty acids, the so-called winterizing method, the raw product is slowly cooled in tanks and stored for a long period of time at the end temperature. A crystal fraction is formed at the bottom of the tank, and in the other portion of the tank a liquid oil fraction is formed. These fractions are then separated by decanting. This method is time consuming and, because of lacking selectivity, not very advantageous with respect to yield.
In another known method, the raw product is mixed with a solvent in high excess and then slowly cooled in an agitator vessel or in a scraper crystallizer. After reaching the end temperature, the crystals are separated from the liquid phase in centrifuges or filters, and by washing the crystals with fresh cool solvent the selectivity is increased. For fractionation of palm oil according to this method acetone or methyl-ethyl-ketone (MEK) is used as solvent. However, it has been found that it is very difficult to obtain low melting oil fractions according to this method. In addition, the use of mechanical means in forming and separating the crystals is very expensive and not very reliable.
It is an object of the present invention to provide a method which does not have the disadvantages of the prior art methods. According to the invention, a method for fractionation of substances by crystallization is provided, said method comprising mixing the raw product, in particular a mixture of fatty acid esters, with a solvent and circulating and cooling the solution in a circuit, with crystals being formed in the solution by gradually lowering the temperature, and characterized in that the crystals are continuously separated from the circulated solution.
The invention also refers to a device for carrying out the method, said device comprising in the circuit a circulation pump, a cooler/heater and a storage tank, and is characterized in that a fractionating column is further provided in the circuit.
The invention relates also to the use of the method for fractionation of mixtures of fatty acid esters, particularly mixtures of glycerides.
It has been found that fractionating of fatty acid esters by crystallisation is particularly effective when the raw product is mixed with a solvent of the alkane sequence, preferably hexane (alkane C.sub.6) and is slowly cooled, the crystals formed during cooling being continuously separated from the liquid phase.
Further, it has been found that this method is particularly easy when the fatty acid ester/solvent mixture is pumped in a circuit through a cooler and a fractionating column. By controlled cooling crystallization takes place, and in the fractionating column the crystals are continuously removed from the circuit.
It has been found that the fractionating column operates particularly effectively when the solution containing the crystals trickles on slightly inclined filter surfaces connected for cross-current and covered with a felt of synthetic fibres. The crystals are kept on the filter surfaces, whereas the solution freed from the crystals drops through and can be collected at the bottom of the column. The filter surfaces are progressively filled and covered with crystals from the top to the bottom. When the upper filter surfaces are filled, the solution containing the crystals are fed in cross-current over these filter surfaces to the lower filter surfaces still being active. These filter surfaces are finally also covered with crystals.
It has been found that after reaching the end temperature corresponding to the desired fractionation the circuit can be interrupted and the oil/solvent fraction can be drawn off. By circulating fresh solvent and cooling at the same temperature the crystal fraction in the fractionating column can be washed effectively one or several times. The collected washing solution can be withdrawn after each washing and be added to the oil fraction or employed as solvent for the raw product or initial product on the next fractionation.
The crystals remaining in the fractionating column can be removed in molten form after adding solvent and circulating the solvent under heat supply.